1. Technical Field of the Invention
The present invention relates to a unit for sensing physical quantity, and more particularly to a unit for sensing a hydraulic pressure which has a failure diagnosis function for an abnormal power voltage.
2. Background Art
In general, a hydraulic brake system mounted on a vehicle is provided with a pressure sensor to sense pressure handled by the system. The pressure sensor unit has, for example, an arrangement to transform pressure in a brake pipe into a voltage signal by a hydraulic pressure sensor and output the voltage signal to an external control unit such as an ECU (Electric Control Unit) through, for example, a buffer circuit and an amplifier. Thus far, to enhance the detection reliability of the pressure sensor unit, a plurality of hydraulic pressure sensors are set up in the same brake pipe system. The voltage differences among the voltages from each hydraulic pressure sensor are sensed to compare the differences with a predetermined value by an external control unit for failure diagnosis of the hydraulic brake system. On the other hand, a recent requirement is for a pressure sensor unit with a failure diagnosis function which has the same level of detecting ability with the plurality of hydraulic pressure sensors.
As one of the pressure sensor units introduced in the above, a pressure sensor unit with the failure diagnosis function to detect a disconnection of a wiring harness (for power line, signal line, and ground line in general), connecting the pressure sensor unit and the external control unit, and defective contact at the wiring harness (for example, linking-up connector part, solder part) has been shipped hitherto. In the pressure sensor unit like the above, one of the arrangement may be like below. For example, the voltage difference between the power line and the ground line is monitored. If the difference becomes lower than a predetermined value, an accident occurrence is informed to the external control unit by forcibly shifting the output voltage of the sensor unit from a normal output voltage range to out of the normal range as an accident signal. However, in this arrangement, when the voltage difference becomes lower than the predetermined value, it is impossible to clarify the reason of the voltage lowering if the lowering is due to the voltage lowering of the power line (by increase of impedance of the power line.), or due to the voltage raising of the ground line (by increase of impedance of the ground line). As a result, if the accident signal is fixed at either a high voltage side or a low voltage side, the accident signal will fall into a normal output voltage range of the pressure sensor, resulting in an evaluation impossibility if the accident has happened or not.
Considering the above issue, the following physical quantity sensor unit has been developed (For example, Japanese Patent First Publication No. 2001-183164). The physical quantity sensor unit includes a power monitoring circuit and an oscillator circuit. The voltage difference between the power line and the ground line is monitored. If the voltage difference becomes lower than a predetermined value, the power monitoring circuit outputs an accident signal, and the oscillator circuit starts operation in response to the accident signal and outputs a high level signal and a low level signal alternately at predetermined periods.
The guaranteed operation voltage of the physical quantity sensor written in the Japanese Patent First Publication No. 2001-183164 is about 3V. Then, if the power voltage is lowered to below 3V, the physical quantity sensor becomes impossible to output the accident signal. As a result, an abnormality of the power voltage caused by a defective contact at a connecting portion of the power line and/or the ground line can not be detected in a wide voltage range, resulting in less reliability of the failure diagnosis function.